Cleaner way to check for successful login

Snapchat returns ‘logged: true’ upon successful API call (in this case
it means successful login) then we set the auth_token. Replacing the
username with the username from Snapchat can be tricky as Snapchat will
sometimes return ‘false’ so just leave that out.
Updating username added

I was wrong about the username not needing to be set by the result from
Snapchat, sorry.
Adjusted login logic